1. Field of the Invention
The present invention relates to a bass-reflex loudspeaker structure. The invention also relates to a method for manufacturing a bass-reflex loudspeaker structure.
The invention is in particular suited for use in bass-range loudspeakers.
2. Description of Background Art
Among the requirements set for loudspeaker enclosures in the reproduction of low-frequency sounds, the most important factors are related to the capability of the enclosure to take internal pressure variations with deformations as small as possible (requiring stiffness) and the freedom of the enclosure from structural resonances. Generally, the enclosure is made from planar panels that are supported and stiffened from within the enclosure. As curved surfaces are structurally much stiffer than planar surfaces, also spherical, cylindrical and other equivalent shapes have been employed in the structures of loudspeaker enclosures.
One basic loudspeaker enclosure construction for reproduction of low-frequency sounds is the bass-reflex enclosure. Herein, the enclosure opening made for the loudspeaker unit is complemented with another opening that frequently has a duct connected thereto. Then, the air contained in the duct forms an acoustic impedance (a mass), while the air contained in the enclosure forms an acoustic capacitance (a spring), and the resonant frequency of this combination is dimensioned to cooperate with the loudspeaker unit. At the lowest frequencies, the combination exhibits a resonance, whereby the resonant circuit acts as a load to the loudspeaker unit. Then the excursion of the loudspeaker unit is small and the major portion of the radiation takes place via the reflex opening. When the goal is to reproduce sound in the very low-frequency range, the resonant frequency of the overall system must be lowered by way of either increasing the volume of the enclosure or the acoustic mass of air contained in the reflex duct. In many cases, a large size of the enclosure becomes a disadvantage that must be avoided, whereby the length of the reflex duct must be made long. Since the radiation occurring at the resonant frequency takes place via the reflex duct, the desired acoustical power output affects the flow velocity of air in the duct. If the flow velocity in the duct increases above a given limit value, the flow becomes turbulent thus evoking sound distortion and compression. Hence, the minimum cross section of the duct is determined by the desired acoustical power output. When a higher acoustical power is desired, the duct cross section area must be increased but the duct becomes longer consequently. A long duct cannot be fitted straight into an enclosure and a conventional approach has been to fold the duct in different angles, but abrupt bends cause turbulence even at small air flow velocities. The dimensioning of a bass-reflex loudspeaker and electrotechnical solutions to the problems thereof have been described widely in the literature of the art and, e.g., in European patent EP 0 322 686.